1. Field of the Invention
The present invention generally relates to the manufacture of semiconductor devices and, more particularly, to the performance of reactive ion etching and the reduction of damage to semiconductor wafers and earlier formed structures during reactive ion etching.
2. Description of the Prior Art
Scaling of electronic element structures to smaller sizes in semiconductor integrated circuits having increased integration density (to obtain improvements in propagation speed, noise immunity and economy of manufacture) has increased the criticality of many wafer processing operations. At the same time, there has also been a trend to increase wafer size to permit process costs to be spread over an increasing number of chips which may compromise the accuracy with which processing can be performed. For example, reactive ion etching is generally considered to be a relatively rapid, well-controlled process useful at numerous stages of integrated circuit manufacture but has presented criticality which has compromised manufacturing yields in modern integrated circuit designs.
During reactive ion etching, a plasma generated by radio frequency (RF) electric field energy and confined by a magnetic field (to increase plasma density and allow reduction of RF energy and bias) is used to develop charged species (electrons and ions). An etchant is introduced into the chamber 15 and ionized and the ions are accelerated toward a wafer by an electric field to etch the surface thereof in a manner well-understood in the art. The speed of the etching process and throughput of the apparatus in which it is conducted is dependent on the density of the plasma adjacent a particular region of the wafer and it is generally desirable to form a plasma with a high density of charged species. By the same token, however, the density of plasma to which the wafer is exposed must, at least on average, be substantially uniform to achieve the same etch rate at all points on the wafer surface particularly for very large diameter wafers.
Reactors for the performance of reactive ion etching must accommodate at least one wafer and thus are of sufficient size to allow local variations and non-uniformity in density of the plasma. Therefore, it has become the practice to use a rotating magnetic field above the wafer to cause the plasma to be repeatedly swept across the surface of the wafer to increase uniformity of exposure of the wafer to the plasma even when the plasma density is not uniform.
However, both increase in size of reactor chambers to accommodate larger wafers and increases in process criticality as electronic elements are scaled to smaller sizes has resulted in damage to wafers due to plasma non-uniformity becoming a significant factor in manufacturing yield. At larger reactor or wafer sizes, a magnetron effect causes a gradient of plasma density across the wafer. The gradient of plasma density, in turn, produces a variation of plasma potential across the wafer because of the tendency of energetic electrons from the plasma to drift in a direction perpendicular to both the magnetic field and the electrical field, referred to as the ExB drift. This drift of electrons causes additional ionization which produces further energetic electrons to contribute to the electron drift and further contribute to the gradient of plasma density across the wafer. It can be understood that this effect will increase for larger reactor and wafer sizes and can become great enough to damage thin gate oxide films in FET arrays and other structures which become very thin when scaled to smaller sizes for higher integration density.
On the other hand, if no magnetic field is used to increase plasma density, an increased bias voltage of about 1 KV or higher is necessary to maintain a sufficient density of plasma above the wafer for reactive ion etching to proceed at an acceptable rate. However, this large bias voltage is sufficient to cause X-ray damage to oxides and lattice damage to the wafer. Therefore, it is the practice to use both a magnetic field and reduced electric field bias for reactive ion etching. However, the ExB drift (and its effect on non-uniform plasma density) is, of course, a function of both electrical and magnetic field strength or intensity and the plasma density developed, which, for a given reactor or wafer size, effectively places a limitation on the magnitude of both the electrical bias and magnetic field which can be used, limiting throughput of the reactor.
An article entitled "Reduction of Charge-up Damage in Magnetron RIE" by Yukimasa Yoshida, published in Electrochemical Society Proceedings, Vol 95-5, pages 236-245, reports that uniform magnetic fields near the wafer surface cause damage and that damage can be reduced by using a non-uniform magnetic field in a magnetron RIE reactor using a rotating plurality of permanent magnets to obtain increased uniformity of plasma density and plasma potential. However, permanent magnets do not allow the magnetic field to be varied to optimize the plasma density or magnetic field gradient in accordance with desired manufacturing process parameters and require substantial mechanical support and mechanisms to rotate them and, hence, are impractical for integrated circuit production. On the contrary, RIE reactors which have become standard in the industry generate rotating magnetic fields by applying varying currents to stationary coils in a sequence to provide a rotating magnetic field. Such arrangement do not readily lend themselves to the simultaneous production of a rotating magnetic field which is also non-uniform since any variation of coil geometry which would create magnetic field non-uniformity would be superposed with other non-uniformities from other coils as the magnetic field was rotated. That is, the gradient in magnetic field strength which might be developed for one coil would not be maintained for a plurality of coils energized by currents of differing phase to obtain field rotation. Further, since the coils currently existing in current commercial reactors constitute an expensive component thereof, alteration of coil geometry is not economically feasible to obtain a suitable non-uniformity of a magnetic field which must also rotate.